This invention relates generally to a reflectometer of the type used to measure certain optically reflective properties of opaque materials. More specifically, the invention relates to a portable, hand held reflectometer which utilizes at least one blue light emitting diode to measure the brightness of paper products. The invention also relates to a paper brightness measuring reflectometer which employs two separate blue light sources of any suitable type arranged so as to substantially reduce measured paper brightness variations caused by changing the orientation of the instrument relative to that of the fibers in the paper whose brightness is being measured.
Broadly speaking, surface reflectance meters have long been known and used in the prior art in a wide range of applications to control the appearance quality of manufactured goods according to set standards and/or purchaser specifications. For example, manufacturers often wish to control the gloss and color characteristics of products having painted surfaces. Magazine and newspaper publishers often desire to purchase paper from paper manufacturers for use in their periodicals according to selected specifications of brightness, as do producers of high quality bond paper such as is used for letterhead. Book publishers are yet another example of paper users who often desire to purchase paper according to selected specifications of appearance. In this way, appearance of the paper in a given book can be maintained from page to page, as well as the appearance of the paper from one book copy to the next in a given printing, as well as the appearance of the paper in the books of different printings, even where such printings occur years apart.
One prior art reflectometer device which is adapted to measure the "gloss" of a painted surface is disclosed in U.S. Pat. No. 4,761,676 issued to G. R. Wiles et al. on Aug. 2, 1988. This device is a portable system which employs an infrared light emitting diode to generate a collimated light beam which is passed, first, through a stationary diffuser and, then, through a rotary light chopper removably disposed in the light beam path, to be focused as a spot of light on the surface whose gloss is to be measured. Light reflected from the surface passes through an imaging lens, thence through a device containing a light slit onto a first photodetector. The rotary chopper contains a series of light chopper blades which periodically interrupt the light beam. One of these blades contains a synchronization mirror which, once during each rotation of the chopper, reflects light from the light emitting diode to a second photodetector. The light signals received by each of these photodetectors are converted to electrical impulse signals which are fed to an electrical signal processor. Upon removal of the light chopper from the light beam path, the processor determines the time rate of change of light flux falling on the first photodetector by measuring the time it takes for reflected light falling thereon to increase from a first to a second predetermined level, which is indicative of the degree of gloss of the surface.
The subject reflectometer, while utilizing light reflected from a sample, is relatively complex and employs moving parts in the operation thereof. Also, it is not adapted to measure the surface reflectance property of brightness, with which the present invention is concerned. Finally, it does not contain a blue light emitting diode essential to an important aspect of the present invention.
Hand held optical photometers which detect light reflected from a test strip inserted therein, which strip serves as a calibration standard, are also known in the prior art. See German Patent No. DE 3026439 A1 published Feb. 11, 1982. So, too, have methods for measuring the quality and composition of substances, materials and coatings, by measuring the brightness of light reflected from a sample at a specific angle, long been known in the prior art. See, for example, Russian Pat. No. SU 1396008 A1. So, too, have portable, hand held photometers for measuring the color of painted surfaces which direct a broad band optical light beam (white light) onto a sample and detect the reflected light from the sample with blue enhanced silicon photodiode receiving devices been known and used in this country as exemplified by the X-Rite 918 Tri Stimulous Colorimeter manufactured by X-Rite of 3100 44th Street S.W., Grandville, Mich. 49418.
Finally, fixed, non-portable brightness testers, which use a blue wavelength filter in association with a broad spectrum light beam generating device (a "white light" generator), and which detect the magnitude of blue light reflected from a sample pad of paper at a selected angle relative to the incident light beam focused on the pad, have long been known and used in the prior art. Such prior art testers have also employed an ultraviolet light generator ("black light" generator), such as a fluorescent or xenon lamp, in addition to a white light generator in the measurement of brightness. But these testors are large and heavy and therefore must be operated from fixed platforms because their optical systems are bulky and heavy. Also, a great deal of power is consumed in their light beam generating systems. It is also necessary to fix the orientation of such brightness testers relative to the alignment of the paper fibers, because changes in this orientation cause substantial changes in the brightness of the paper as sensed by the tester.
By means of our invention, these and other difficulties encountered with prior art reflectometers used in measuring the brightness of opaque materials, such as paper, are substantially overcome.